The present invention is a method of forming a resilient hydrophilic cellulose-based product and the products formed by the method. The method is especially directed to the formation of a sheeted pulp product that can be mechanically formed into a fibrous fluff having increased water absorption rate and water holding capacity. The product is especially useful for making articles such as disposable diapers and sanitary napkins.
Approximately 35 billion disposable infant diapers are presently manufactured worldwide each year. Each diaper contains an average weight of about 50 g of fluffed cellulosic fiber. Worldwide it is estimated that about 2 million tons of wood pulp are used in absorbent products of all types. Besides infant diapers these include sanitary napkins and tampons, adult incontinent pads and numerous other products such as bed pads, wipes and towelettes. All of the above products have had a rapid technical evolution over the past decade. However, little or nothing has been changed in the wood pulp products which are fiberized to form the essential absorbent fluff in this group of products. It is evident that there is an enormous market potential for fluff pulps that would be more effective absorbents if they could compete economically with presently available materials.
It has been known for many years that it is possible to make cellulosic materials for absorbent fluffs which have more rapid liquid pickup and greater liquid retention under pressure. As one example, U.S. Pat. No. 3,224,926 to Bernardin describes treatment of a cellulosic material, such as sheeted wood pulp, with a crosslinking agent such as formaldehyde or dimethylolurea. The impregnated pulp is dried to about 10% moisture, then fluffed. After fluffing, it is heated to 110.degree.-150.degree. C. for a sufficient time to effect curing. The fiber is said to be useful for making articles such as filter media, cushioning, and absorbent products such as sanitary napkins and wipes.
In a somewhat related process, the same inventor in U.S. Pat. No. 3,434,918 describes a paper sheet in which a portion of the furnish has been treated with a crosslinking agent such as those just noted. Preferably one portion of the fiber is treated in sheeted form with a crosslinking agent and curing catalyst. This is wet aged to insolubilize the crosslinker, then redispersed and mixed with untreated fiber. The mixture is again sheeted on a papermaking machine. As part of this process, the sheet is subsequently creped in a conventional manner on a Yankee dryer. Final cure of the product is effected by raising the sheet temperature to about 150.degree. C. for a short period of time. Finished sheets are suggested for use as wipes, toweling and tissues and may be further useful in diapers.
In another patent to the same inventor, Can. 813,616, the invention is directed toward a creped sheet having nearly circular absorption properties. The furnish is a mixture of collapsed fiber, which is dry crosslinked, and conventional fibers. The crosslinked fiber must be treated, air dried, fluffed and heat cured at about 130.degree.-135.degree. C. prior to mixing with the conventional papermaking fibers.
Van Haaften in Can. 806,352 discloses an invention directed to pads, such as sanitary napkins and diapers, which contain a layer of crosslinked fiber overlying or surrounding a second layer of conventional cellulose fluff. This construction is said to give better flow control and reduce surface spreading of wetness.
The above noted and other crosslinking materials have long been used for treatment of textiles for giving properties such as wrinkle and crease resistance. Virtually all of the popular wash and wear fabrics contain cellulosic fiber which has been crosslinked in some manner. Welch, in U.S. Pat. No. 4,472,167, describes treatment of cotton textiles with glyoxal and glycols. A glyoxal and a glycol are co-reacted with cellulose in a the presence of an aluminum sulfate catalyst and an alpha-hydroxyacid; e.g., tartaric or citric acid. The latter material serves as a catalyst activator. The treated textile must contain at least 30% cellulose fiber, but the invention is also said to apply to nonwoven textiles and paper. A critical part of the invention is drying the product and heating to a temperature of 110.degree.-135.degree. C. for 0.5-5 minutes.
Noller, Chemistry of Organic Compounds, 3rd Ed.: 847, W.B. Saunders Company, Philadelphia (1965) notes that glyoxal can be used for shrinkproofing rayon fabrics. This apparently works by crosslinking cellulose chains through the formation of cyclic acetals. Glyoxal is also said to increase the wet strength and absorbency of paper. A heat curing step above 100.degree. C. under dry conditions is implicit in the above treatment.
Tesoro and Willard in Cellulose and Cellulose Derivatives, Part V, Norbert M. Bikales and Leon Segal, eds.: 837-838, Wiley-Interscience, New York (1971) also not that glyoxal can be reacted with cellulose under conditions similar to those used for the reaction of formaldehyde. Glyoxal and other dialdehydes were tried, apparently unsuccessfully, to offset the serious strength losses associated with formaldehyde crosslinking of cellulosic fabrics.
While processes direct to making crease resistant fibers have been very successful commercially, none of the others described above appear to have advanced beyond the stage of laboratory curiosities. This is particularly the case for treatments involving wood pulp. One or both of two major stumbling blocks have remained in the way of commercial acceptance of so-called crosslinked fibers for use in absorbent products such as diapers. In all cases a heat curing step carried out well above 100.degree. C. for some significant length of time has been required. In some cases this has involved heat treatment of a sheeted product and in other cases, heat treatment of a sheet product which has subsequently been fluffed. To date there has been no practical way found to do this on a paper machine without a major loss in production capability. Similarly, the problem of heat curing treated fluff at the point of production or point of use has not been resolved. A further problem was only hinted at in the above cited article by Tesoro et al. This is the serious matter of embrittlement and discoloration of crosslinked fibers. To the present time no fibers which have been crosslinked in sheeted form have been suitable for subsequent conversion to fluff because of serious fiber breakage during the fluffing process. This results in an unacceptable amount of fine material. The problem of fluffing an uncured pulp and subsequently heat curing in a diaper plant was mentioned previously. The additional burden and expense that heat curing a fluff pulp would place upon a diaper manufacturer is one that to date they have been unwilling to assume. For these reasons, despite their known advantages, crosslinked pulps have found essentially no commercial application in absorbent sanitary products.
It might be noted that there is still considerable controversy among cellulose chemists as to whether materials such as dimethylolureas or glyoxal actually serve as crosslinking or briding agents between adjacent cellulose molecules or whether some other reaction occurs. Whatever the nature of the reaction, it is immaterial to the present invention where all of the problems noted above have been successfully overcome in a convenient and economical manner.